Hi,
There are no known limitations with respect to customization of Workspace Desktop Edition. According to Supported Operating Environment "Genesys Virtualization Platform Support" Workspace Desktop Edition (Interaction Workspace User Interface) supports XenApp 6.5 version starting from 8.1.x version.
If you want to know about limitations I would advise you to consider the Workspace Desktop Edition Release Notes periodically as they contain the sections dedicated to such kind of information.
Roman Tereschenko